Our invention relates to improvements in a continuously variable belt and sheave drive for automotive vehicles such as the infinitely variable drives disclosed in patent publication H398 by Lemieux and Fisher, Eichenberger U.S. Pat. No. 4,876,920 and Hahne U.S. Pat. No. 4,583,423. Each of these patents is assigned to the assignee of our invention.
It is known in the automatic transmission art to provide a torque delivery driveline for a vehicle having an internal combustion engine wherein a wide range of ratios can be obtained to permit the vehicle engine to operate at its most efficient speed consistent with its speed torque characteristics. With a conventional driveline using multiple ratio gearing the engine is allowed to operate at each of several speeds depending upon the gearing ratio that is selected. With such geared transmissions, the vehicle operator controls intake combustion air delivery by controlling the engine throttle opening. Each gearing ratio then is characterized by a variety of engine speed and throttle opening combinations as vehicle operator attempts to match the driveline performance with varying driving demands.
The engine in such known drivelines is capable of achieving its optimum horsepower output throughout only a limited range of speeds. Thus, the fuel consumption rate will differ if the operating demands require an engine speed that is less than or more than its optimum speed. Energy efficiency is sacrificed to achieve improved performance.
It is known practice to provide an infinitely variable transmission in a driveline in combination with an internal combustion engine so that torque can be delivered to the traction wheels with a continuously variable ratio. Such infinitely variable transmissions typically include a control system for establishing an actual engine speed that is consistent with a so-called target speed at which the engine will operate with its most efficient specific fuel consumption. Improved fuel efficiency then can be achieved throughout the entire range of vehicle speeds made available by the infinitely variable driveline.
It is known that for each engine net power output point, there will exist a throttle position at which the specific brake fuel consumption for the engine will be a minimum. There are numerous prior art teachings that recognize this characteristic of matching an infinitely variable transmission with an internal combustion engine to take advantage of the capability of a continuously variable transmission to improve the driveline efficiency. This eliminates the limitations that are characteristic of a transmission having multiple fixed ratios. Examples of such prior art teachings are found in U.S. Pat. Nos. 4,546,673 and 4,683,779.
Operation of such a driveline with a controller that will command a minimum brake specific fuel consumption over a broad speed range will result in a noticeable difference in drivability, compared to drivelines with multiple, fixed ratios, the vehicle operator as the driveline responds to changes in operator demands for performance. For example, the engine speed will not increase upon advancement of the engine throttle until the control system for the driveline makes adjustments that are consistent with the previously described optimum relationship between engine throttle position and engine power. A simple demand for more torque will not necessarily result in a change in engine speed that is consistent with an increasing vehicle speed.
Prior art mechanisms also lack an adequate teaching of means for achieving a proper clamping force on the sheaves to prevent slippage of the belt during rapid changes in the torque demanded by the vehicle operator.